While the present invention involving a paper folding operation can be effectively used in a plurality of different media printing or booklet-making configurations, it will be described for clarity as used in electrostatic marking systems such as electrophotography.
By way of background, in marking systems such as Xerography or other electrostatographic processes, a uniform electrostatic charge is placed upon a photoreceptor belt or drum surface. The charged surface is then exposed to a light image of an original to selectively dissipate the charge to form a latent electrostatic image of the original. The latent image is developed by depositing finely divided and charged particles of toner upon the belt or drum photoreceptor surface. The toner may be in dry powder form or suspended in a liquid carrier. The charged toner, being electrostatically attached to the latent electrostatic image areas, creates a visible replica of the original. The developed image is then usually transferred from the photoreceptor surface to a final support material such as paper and the toner image is fixed thereto to form a permanent record corresponding to the original.
After fusing of the toner image onto the paper substrate, the paper is either moved to a collection tray where it is removed from the marking apparatus, or it may be transferred to a finishing station for further processing. At the finishing station, the copies may be compiled into sets, or may be stapled, or may be directed to a booklet maker where sets are compiled, saddle stapled and folded to make booklets such as Signature Booklets. “Signature Booklets” are made from multiple sheets of paper or similar printable media where up to four images or pages are placed on each sheet [two per side] such that when the sets are compiled, stapled and folded in half, that the images appear in proper page order. The signature booklet may employ an outer cover sheet compiled during the set making process. The cover is often a heavier paper basis weight and or rougher texture than the ordinary pages in the booklet. The basis of embodiments of this invention involve a knife folder structure useful to provide more precise and more reliable folding operations, especially when creating thicker booklets. Some Signature Booklet Makers (SBM) can create from 8 to 200 imaged page signature booklets, requiring 2 to 50 signature sheets per booklet. For example, the following Signature Booklet Maker Finishers, typical of office or light production reprographic equipment are capable of producing Signature Booklets sizes as indicated:
ManufacturerSBM FinisherSheets/SetPages/BookletXeroxMFF, HCSS, HVF1560RicohBK50001560CanonV-22080RicohBK501030120Konica-MinoltaSD-50150200
Therefore, the folder assembly of the present invention must be able to accommodate sets comprising at least 2 and up to 50 signature sheets, and even more for eventual future faster systems. The embodiments of the disclosed invention provide a system which directly measures the thickness of the set to be folded at the folding location and then automatically adjusts the gap between the folding cylinders to reliably accommodate these varied set thickness.
In U.S. Pat. No. 4,643,705 by the same inventor and assignee as the presently described invention, an improved knife folder is defined and claimed. The present embodiments involve a further refinement and substantial improvements over this prior art patent. The disclosure of U.S. Pat. No. 4,643,705 is incorporated herein by reference.
Signature Booklet Makers (SBM's) of the prior art accept 4-up signature sheets (2+2 images per signature), compile individual sheets into a set, saddle staple the sheets and then fold the stapled sets to make signature booklets. Optional face trimming shears off the shingled thumb edge and produces a high quality customer-ready finished product. The folding in the prior art of the stapled set is typically accomplished by a knife folder as follows:                A blade is centered over a cylindrical folder nip        The stapled set is staged and centered over the cylindrical nip        The blade is actuated        The blade bends the set        Friction between the cover sheet and the folding cylinders drives the set into the fold nip        The nip acquires the set and folds it along the staple axis.        
One typical prior art failure mode with SMB knife folders is the “sucking” or tearing of the front cover or outer most sheet off of the set. This occurs when the drive forces on the cover sufficiently exceed the drive forces on the balance of the set.
Another prior art failure mode occurs when the set is not acquired symmetrically and the staple axis is mispositioned relative to the “fold line” (i.e. the right/left margins). This increases the level of shingling and shifts image registration relative to the fold line. Both of these failures can be related to the sheet to sheet interfacial forces or stresses (level and uniformity) as the set is acquired by the knife folder cylinder nip.
The market place's growing desire for thicker booklets is only constrained by the machinery's ability to produce quality booklets in a compact, cost effective module.